This application claims the priority benefit of Japanese application serial no. 2001-243851, filed on Aug. 10, 2001.
1. Field of the Invention
This invention relates generally to an image forming device for a copy machine, a printer, a facsimile, or a multi-function machine having the above functions. More particularly, the invention relates to an image forming device for performing an image process by using a magnetic force, so that no xe2x80x9cvoids in the back endxe2x80x9d occurs, the fine-line reproduction can be further improved, and the carrier adhesion can be further avoided.
2. Description of Related Art
In general in the image forming device using the electronic photography, such as the copy machine, the printer, or the facsimile, etc., an electrostatic latent image, which corresponds to an image information, is formed on a latent image supporter consisting of a photo-sensing drum or a photo-sensing belt, and a developing process is executed by a developing device to obtain a visualized image. Regarding the electrophotography, conventionally, a developing process, using a magnetic brush that makes the developer nap in a brush chain shape on the developer supporter, is well known
In the above magnetic brush development, a good image concentration can be obtained if a distance between the latent image supporter and the developer supporter at a developing region is very close to each other. In addition, not much edge effect is known, but if too close, it is known that an image-quality degradation, the so-called xe2x80x9cvoids in a back endxe2x80x9d, will occur, i.e., white spots occur is the back end of a black solid image or a half-tone solid image.
In order to solve the above xe2x80x9cvoids in a back endxe2x80x9d issue, the inventor has proposed a magnetic-brush developing device that narrows a nip portion (see Japanese Laid Open 2000-305360). According to the proposed developing device, in order to reduce the xe2x80x9cvoids in a back endxe2x80x9d, one of the methods to narrow the developing nip is to decrease a half-value width of a developing magnetic pole (below 22xc2x0). The half-value width is an angular width corresponding to a value that is half of the maximum normal magnetic force in magnetic force distribution curve along the normal direction of the developing magnetic pole. For example, if a maximum normal magnetic force of a magnet made of N-pole is 120 mT, the angle width corresponding to a value of 60 mT is the half-value width, which is also termed a peak 50% half-value width However, as the half-value width of the developing magnetic pole is reduced, it is understood that a carrier adhesion occurs easily. For example, if the half-value width is less than 20xc2x0, the carrier adhesion is very obvious.
According to the foregoing description, an object of this invention is to provide a developing device, an image forming device and a process unit, which can prevent voids in a back end from occurring to improve the fine line reproduction, and additionally, the carrier adhesion can be also prevented.
Accordingly, for achieving at least the above object(s), the invention provides a developing device. The developing device comprises at least a developer supporter for supporting a developer; a latent image supporter, opposite to the developer supporter, for supporting a latent image; and a napping unit. The napping device is used for napping the developer on the developer supporter by magnetic poles. The napping unit fiber comprises a developing main magnetic pole installed at a location opposite to the latent image supporter, and a developing auxiliary magnetic pole located at the downstream side of the developing main magnetic pole in the rotational direction of the developer supporter. In addition, the developing main magnetic pole has a half-value width equal to or less than 22xc2x0. The composite magnetic flux density of the developing main magnetic pole and the developing auxiliary magnetic pole is equal to or greater than 80 mT.
According to one aspect of the present invention, preferably, when the developer supporter is located opposite to the latent image supporter, with respect to the connection line of centers of the two supporters, the peak of the magnetic flux density in the normal direction of the developing main magnetic pole is deviated by 2xc2x0xcx9c10xc2x0 towards an upstream side of the developer supporter in the rotational direction of the developer supporter. In addition, the developing auxiliary magnetic pole has a preferred half-value width equal to or greater than 35xc2x0. if the ratio of the half-value widths of the developing main magnetic pole and the developing auxiliary magnetic pole is 0.4xcx9c0.6 and the ratio of the peaks of the magnetic flux densities of the developing main magnetic pole and the developing auxiliary magnetic pole is 0.7xcx9c1.2, the developing device can be more effective.